a) Field of the Invention
The present invention relates to a vari-focal lens system, and more specifically a vari-focal lens system which is suited for use with video cameras.
b) Description of the Prior Art
It is strongly demanded in these days to configure zoom lens systems for video cameras such that they are compact. Further, it is required to configure the zoom lens systems for video cameras so as to have high vari-focal ratios and wide field angles at wide-angle positions thereof. However, conventional zoom lens systems are not sufficiently satisfactory as exemplified by the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 5-60,973 which is fairly compact but has a zoom ratio of approximately 8 and a field angle 2.omega. of 55.degree. at a wide-angle position thereof. This conventional zoom lens system has a total length of approximately 19 times that of the product of the focal length at the wide position multiplied by the tangent of half the field angle thereof. This zoom lens system consists, in order from the object side, of a first lens unit which has a positive refractive power and is always kept stationary, a second lens unit which has a negative refractive power and is movable for varying a focal length of the vari-focal lens system, a third lens unit which has a positive refractive power and is always kept stationary, and a fourth lens unit which has a positive refractive power and is movable for correcting a deviation of a focal point of the vari-focal lens system which is caused due to the variation of the focal length or a shift of an object point. A zoom lens system which is composed as described above has a defect that a total length of the zoom lens system and a diameter of a front lens component used in the lens system are remarkably large when the lens system is configured so as to have a wide field angle.